This invention relates to preforms for molding processes, especially resin transfer molding processes and to methods for preparing the preforms.
Preforms for resin transfer molding (RTM) are typically composed of layers of oriented fiber material which are assembled and shaped prior to insertion into a mold for resin injection. Resin is then injected into the mold to infiltrate between the fibers to fill out the composite part. This process for producing composite parts is known as "resin transfer molding" (RTM).
In such a process, it is difficult to incorporate a layer of unidirectional fibers, because there is no way to maintain dry unidirectional fibers in register during preform assembly. It is also difficult to place the fibers in a preform because of the tendency of the fibers to separate. Even if one were to successfully place a layer of dry unidirectional fibers in the preform, it is likely that during resin injection, the fibers will separate and shift, thus reducing part strength, and making it difficult to maintain part to part consistency.
To overcome this problem, a preimpregnated unidirectional fiber preform was used which has a resin content of about 35 percent by weight. While sufficient to hold the fibers together, this preform failed to allow impregnation of the preform with matrix resin during the molding process. On the other hand, a unidirectional fiber perform having a low resin content (3 to 5 percent by weight), such as that described in U.S. Pat. No. 5,496,602, was found to be difficult to handle, resulting in fiber breakage.
Dry fiber preforms, in which the unidirectional fibers are either stitched together or held together by a small fraction of transverse fibers, are an alternative to preimpregnated fiber preforms. However, stitching reduces the toughness of the composite and the dry fibers can be difficult to wet out.
It would be desirable to provide a unidirectional fiber preform having a resin content which is sufficient to hold the fibers in the desired shape and position but small enough to leave the resulting preform porous so that it can be impregnated with matrix resin during subsequent molding processes. Additionally, it would be desirable to provide a unidirectional fiber preform having improved permeability with respect to the RTM resin to reduce injection time and improve fiber wet out.